In modern semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques of such devices. Conventionally, a flip-chip attachment method has been used in the packaging of IC chips. In the flip-chip attachment method, instead of attaching an IC die to a lead frame in a package, an array of solder balls is formed on the surface of the die. The formation of the solder balls is normally carried out by an evaporation method of lead and tin through a mask for producing the desired solder balls. However, the evaporation method has many drawbacks such as inefficient use of material, degraded yields when the pitch falls below 225 μm, non-stable evaporation masks for 300 mm wafers.
Another method for depositing solder balls is solder paste screening. However, with the recent trend in the miniaturization of device dimensions and the reduction in bump-to-bump spacing (or pitch), the solder paste screening technique becomes impractical. For instance, one of the problems in applying solder paste screening technique to modern IC devices is the paste composition itself. Pastes are generally composed of a flux and solder alloy particles. The consistency and uniformity of the solder paste composition become more difficult to control with a decreasing solder bump volume.
A possible solution for this problem is the utilization of solder pastes that contain extremely small and uniform solder particles. However, this can only be done at a high cost penalty. Another problem in using the solder paste screening technique in modern high density devices is the reduced pitch between bumps. Since there is a large reduction in volume from a screened paste to the resulting solder bump, the screen holes must be significantly larger in diameter than the final bumps. The stringent dimensional control of the bumps makes the solder paste screening technique impractical for applications in high density devices.
A more recently developed injection molded solder (“IMS”) technique attempted to solve these problems by dispensing molten solder instead of solder paste. However, problems have been observed when the technique is implemented to wafer-sized substrates. U.S. Pat. No. 5,244,143, which is commonly assigned to International Business Machines Corporation, discloses the injection molded solder technique and is hereby incorporated by reference in its entirety. One of the advantages of the IMS technique is that there is very little volume change between the molten solder and the resulting solder bump. The IMS technique utilizes a solder head that fills boro-silicate glass molds that are wide enough to cover most single chip modules.
The IMS method for solder bonding is then carried out by applying a molten solder to a substrate in a transfer process. When smaller substrates, i.e., chip scale or single chip modules are encountered, the transfer step is readily accomplished since the solder-filled mold and substrate are relatively small in area and thus can be easily aligned and joined in a number of configurations. For instance, the process of split-optic alignment is frequently used in joining chips to substrates. The same process may also be used to join a chip-scale IMS mold to a substrate (chip) which will be bumped. One problem with current IMS systems is that they are restricted to linear deposition of solder into rectangular molds. That is, the mold and the solder head are moved linearly with respect to each other such that the cavities move perpendicular to a slit in the solder head thereby filling the cavities as they pass. The molds are limited to a rectangular configuration.
The mold materials used for IMS to date have included borofloat glass, silicon wafers, kapton, and polyimide on glass, carbon, and recently molybdenum. Each of these has advantages and disadvantages. The most commonly used material to date has been borofloat glass, because of its durability, ease of alignment, excellent temperature coefficient of expansion, and ability to easily make the proper form-factor (rectangular and thick enough to provide rigidity).
One problem with several of the mold materials, including borofloat, kapton, polyimide on glass, is that an existing infrastructure for building the molds does not exist. Unlike glass masks used in plating of solder, or metal masks used in evaporation of solder, both of which are readily available internationally, mold fabrication does not exist in mass production. While infrastructure does exist for some mold fabrication, such as molybdenum, this suffers from other disadvantages.
A problem common to the solder ball forming techniques discussed above and other techniques not discussed such as molten solder screening is with the mold used for transferring solder balls to substrates. Current molds comprise a pattern of cavities with a one to one correspondence to pads on a substrate, that is, cavities on a mold only exist at locations corresponding to a location on a substrate with a pad. In other words, current molds can only be used for a particular substrate design. Every new or change in design requires a new build of a mask or mold. This is true for existing technologies of plating and evaporation, as well as with IMS. In most cases, it is also preferable to produce multiple copies of masks or molds for throughput or redundancy. The costs for these new masks and molds will vary significantly, but are costly. This also drives delivery time, which can gate the delivery of final parts. This is especially costly in the event of a redesign, which can take several weeks onto the delivery schedule.
Therefore a need exists to overcome the problems with the prior art as discussed above.